Unravelling Forest Complexity: Resource Use Efficiency, Disturbance, and the Structure-Function Relationship.
Reduced structural equation modeling (SEM) diagrams at canopy structural complexity (CSC) metric calculation resolutions of 0.25 m (a), 2 m (b). CSC predictor variables are on the far left, mediating variables (light use efficiency (LUE) and water use efficiency (WUE)) are in the center, and the response variable, gross primary productivity (GPP) is on the far right. Standardized regression coefficients are shown in gray circles. Blue lines indicate a statistically significant (p < 0.01) mediation effect between mediating and predictive variables, while orange lines indicate that mediation either does not exist or is not statistically significant.
Abstract
Structurally complex forests optimize resources to assimilate carbon more effectively, leading to higher productivity. Information obtained from Light Detection and Ranging (LiDAR)-derived canopy structural complexity (CSC) metrics across spatial scales serves as a powerful indicator of ecosystem-scale functions such as gross primary productivity (GPP). However, our understanding of mechanistic links between forest structure and function, and the impact of disturbance on the relationship, is limited. Here, we paired eddy covariance measurements of carbon and water fluxes from nine forested sites within the
10 × 10 km CHEESEHEAD19 study domain in Northern Wisconsin, USA with drone LiDAR measurements of CSC to establish which CSC metrics were strong drivers of GPP, and tested potential mediators of the relationship. Mechanistic relationships were inspected at five resolutions (0.25, 2, 10, 25, and 50 m) to determine whether relationships persisted with scale. Vertical heterogeneity metrics were the most influential in predicting productivity for forests with a significant degree of heterogeneity in management, forest type, and species composition. CSC metrics included in the structure-function relationship as well as driver strength was dependent on metric calculation resolution. The relationship was mediated by light use efficiency (LUE) and water use efficiency (WUE), with WUE being a stronger mediator and driver of GPP. These findings allow us to improve representation in ecosystem models of how CSC impacts light and water-sensitive processes, and ultimately GPP. Improved models enhance our capacity to accurately simulate forest responses to management, furthering our ability to assess climate mitigation strategies.
Citation
Murphy, B., May, J., Butterworth, B., Andresen, C., Desai, A.R., 2022. Unravelling Forest Complexity: Resource Use Efficiency, Disturbance, and the Structure-Function Relationship. Journal of Geophysical Research: Biogeosciences, 127, e2021JG006748, doi:10.1029/2021JG006748
*** Undergrad first author ***